As the information technology has evolved, communication systems in which information processors such as computers are connected by WLANs (wireless local area networks), Bluetooth™, or other wireless communication means to send and receive files, data, and other information and share information have been built.
Motion picture data, music data, and other large-capacity data are sent and received by utilizing a device which is equipped with broadband communication functions such as a communication device capable of broadband communications using Bluetooth or the like, the communication device being installed in a cell phone or personal computer. Meanwhile, in data communications relying on broadband communications, encryption processing for encrypting data is generally introduced to reduce the danger of eavesdropping of data by a third-party's device equipped with broadband communication functions, forgery of data, and other security dangers even in broadband communications.
However, in order to execute encryption processing in broadband communications typified by Bluetooth, it is first necessary to identify the communicating party. Because communicable areas of broadband communications are wide, there is the possibility that an unexpected device equipped with broadband communication functions is identified as the communicating party.
Even if the communicating party is identified, it is necessary to share session keys for starting a more secure communication between both communicating parties. Because of the characteristics that the communicable areas of broadband communications are wide, when session keys are sent to a communicating party, there is the possibility that the keys are eavesdropped and decrypted by a broadband communication function-equipped device used by a third party. Therefore, it is necessary to enhance the strength of the session keys (e.g., increase the key length). For this purpose, it has been necessary that the broadband communication function-equipped device have high processing capabilities.
Accordingly, it has been proposed to enable session keys to be shared securely between both communication devices (see, for example, patent reference 1).
Communications disclosed in patent reference 1 are now described briefly with reference to FIG. 1. Devices performing communications are herein referred to as an initiator and a target, respectively.
The initiator creates and sends a key in step S11. The initiator executes polling (processing for inquiry), for example. If a response to the polling is sent in from the target, the initiator receives the response and creates an encryption key and a decryption key. The initiator sends the created encryption key to the target.
In step 21, the target receives the encryption key. In step S22, the target generates random numbers. The generated random numbers are taken as a session key. The session key is encrypted using the received encryption key as a key. The encrypted session key is sent to the initiator.
In step S12, if the initiator receives the encrypted session key from the target, the initiator decrypts the key with a decryption key and derives the session key. In step S13, communications using the decrypted session key are started.
If the initiator receives the encrypted session key in this way, the initiator decrypts the encrypted session key with the already generated decryption key, thus obtaining the session key. In this way, the common session key is shared between the initiator and the target. Communications using the session key, e.g., communications in which data sent and received with the session key are encrypted, are performed.    Patent reference 1: JP-A-2006-14076